pgsql: Track statement entry timestamp in contrib/pg_stat_statements

Track statement entry timestamp in contrib/pg_stat_statements

This patch adds 'stats_since' and 'minmax_stats_since' columns to the
pg_stat_statements view and pg_stat_statements() function.  The new min/max
reset mode for the pg_stat_stetments_reset() function is controlled by the
parameter minmax_only.

'stat_since' column is populated with the current timestamp when a new
statement is added to the pg_stat_statements hashtable.  It provides clean
information about statistics collection time intervals for each statement.
Besides it can be used by sampling solutions to detect situations when a
statement was evicted and stored again between samples.

Such a sampling solution could derive any pg_stat_statements statistic values
for an interval between two samples with the exception of all min/max
statistics. To address this issue this patch adds the ability to reset
min/max statistics independently of the statement reset using the new
minmax_only parameter of the pg_stat_statements_reset(userid oid, dbid oid,
queryid bigint, minmax_only boolean) function. The timestamp of such reset
is stored in the minmax_stats_since field for each statement.
pg_stat_statements_reset() function now returns the timestamp of a reset as the
result.

Discussion: https://postgr.es/m/flat/72e80e7b160a6eb189df9ef6f068cce3765d37f8.camel%40moonset.ru
Author: Andrei Zubkov
Reviewed-by: Julien Rouhaud, Hayato Kuroda, Yuki Seino, Chengxi Sun
Reviewed-by: Anton Melnikov, Darren Rush, Michael Paquier, Sergei Kornilov
Reviewed-by: Alena Rybakina, Andrei Lepikhov

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/dc9f8a798307244d791b71f666f50de319799e7c

Modified Files
--------------
contrib/pg_stat_statements/Makefile                |   2 +-
.../expected/entry_timestamp.out                   | 159 +++++++++++++++++++++
.../pg_stat_statements/expected/oldextversions.out | 116 ++++++++-------
contrib/pg_stat_statements/meson.build             |   1 +
.../pg_stat_statements--1.10--1.11.sql             |  23 ++-
contrib/pg_stat_statements/pg_stat_statements.c    | 119 +++++++++++----
contrib/pg_stat_statements/sql/entry_timestamp.sql | 114 +++++++++++++++
contrib/pg_stat_statements/sql/oldextversions.sql  |   4 +-
doc/src/sgml/pgstatstatements.sgml                 |  69 +++++++--
9 files changed, 511 insertions(+), 96 deletions(-)